It is well known that a residual compressive stress at a component surface can increase the strength and fatigue life of the component. For example, residual compressive stresses imposed on inner surfaces of tubes can provide resistance to fatigue and inhibit crack initiation and the rate of crack propagation. Such results are possible since the propagation of a crack requires tensile stresses to be present at the tip of the crack, and if a surface is under compression, the compressive stress must be overcome—in addition to any tensile stresses required for crack initiation and/or crack propagation—before the crack can be created or propagate if already present.
One process to produce residual compressive stress to a surface is called autofrettage. Autofrettage is a process wherein pressure is applied within a container, e.g. a tube, such that the outer surface of the container undergoes elastic deformation whereas the inner surface undergoes elastic plus plastic deformation. After the pressure is removed, the outer surface recovers the elastic strain but the inner surface recovers only the elastic strain with the plastic strain resulting in a residual compressive stress being present.
Autofrettage plastic deformation to the interior of a tube can be created in a number of ways, including the use of explosives, hydraulic pressure, or mechanical force. For example, mechanical autofrettage uses a press to force an oversized mandrel through a tube, thereby causing the inner surface of the tube to yield in tension while the material at the outer surface of the tube remains elastic. After the mandrel has passed through the tube, relaxation of the material results in a distribution of residual stress that is compressive on the inner surface.
In addition to mechanical autofrettage, hydraulic autofrettage can be accomplished by placing a fluid within a sealed container and applying pressure. Looking at FIGS. 1-3, a pump 20 can apply pressure to a hydraulic liquid 40 within a tube 10. The tube 10 has an outer surface 12 and an inner surface 14. Also included are high-pressure seals 30 which prevent the liquid 40 under high pressure from escaping the tube 10. An internal pressure pi within the interior of the tube 10 can cause elastic strain εod on the outer surface 12 and elastic plus plastic strain εid on the inner surface 14 as illustrated in FIG. 2. After the pressure is removed, the elastic strain is recovered at the outer surface 12 and inner surface 14, however the plastic strain at the inner surface 14 results in the presence of a residual compressive stress σid. In this manner, a tube 10 having residual compressive stress at the inner surface 14 is provided.
Although such processes have been used to produce autofrettage within tubes such as gun barrels, processing equipment, high-pressure pump cylinders, and the like, the mechanical autofrettage process requires the manufacture of mandrels that are specially designed and dimensioned for the particular tube, gun barrel, etc. In. addition, the use of hydraulic autofrettage requires the use of high-pressure seals to be attached to the tube, gun barrel, etc., and with equipment failure can result in the rapid and/or uncontrolled release of high-pressure fluid. Such a release is dangerous and requires additional safety equipment to be used with hydraulic autofrettage systems. Therefore, an improved process that provides for autofrettage would be desirable.